Zeus 75 Power Amplifier

Introduction

Zeus 75 Prototype Amplifier & PSU

Amplifier Schematic - STW34NB20 version

Click here for PDF of schematic for printing.

N.B. Prototype uses L200 bias current source, which has better stability.

Standard components for the amplifier

R1-2 are 470 ohm non-inductive 1/4 watt resistors (note: do NOT use lower ohmic values, particularly for low Crss mosfets).

R3 is selected by tuning first with a 470K pot depending on the transformer specification, then is replaced with a fixed non-inductive 1/4 watt resistor. With the Sowter input transformer and a driving source impedance of c. 50 ohms the value of RTerm is about 120K.

Z1-2 are 12V Zeners to protect Q1-2 These MUST be fitted as shown otherwise the mosfet gates can be driven over range and the mosfets WILL be destroyed if this happens.

VBias circuit

Fit Q3 to the heatsink near Q1 and Q2. Also fit LM317 to heatsink to use it's thermal shutdown feature (note v-max for this part - use a pre regulator or separate lower voltage power supply if the main supply is going to be higher than 34 volts).

Alternative is a ST part type L200 regulator in current mode. Note [Jan 2005] the L200 has much better regulation against power supply variation then the LM317, and is to be preferred.

Over Temperature Protection

Also mount a normally open thermal switch (not shown on the schematic) on the heatsink so arranged as to short out the Schottky diode and thus reduce the bias current at a predetermined temperature (50° Centigrade recommended).

Variations

There are several different options which can be built, most of which come down to the required power output as determined by speaker efficiency and listening room size.

My original mono-blocks are low quiescent current amplifiers for 12 or 35 watts which can be constructed with modest specification components and a relatively small heatsink. The choice of power output and distortion can be traded by the use of a 4:1 or 2:1 winding configuration.

A high quiescent current amplifier which will give reduced distortion for the higher power outputs (e.g. 75 watts into 8 ohms), will need a slightly larger power supply but a substantially bigger heatsink, is detailed here.

N.B. For higher bias currents it is recommended to use a tracking bias voltage supply as detailed above, rather than a simple regulated voltage and a potentiometer.

Concept Design

Rendered Image of Zeus 75 Amplifier (only major components shown).

Mono-block Amplifier with separate PSU in its own steel enclosure (this cuts down on the magnetic field radiation/pickup).

The amp is designed to be tall and not deep so as to sit behind or to one side of the speaker it is driving.

Illustrated with an Aavid Thermalloy heatsink profile OS245, size 300 x 200 x 80 mm (note 250 mm is recommended and used in the prototype).

Steel chassis is 300 x 150 x 80 mm which sits in front of and at the bottom of the heatsink. The output transformer sits on top of the steel chassis, the input transformer is inside and thus screened from electrical and magnetic fields (including the output transformer's).

The PSU is in a separate 300 x 200 x 80 mm enclosure and sits under or to one side of the amplifier. Power connection (not shown in this render) is a short loop of cable with 4 pin Cliff connectors.

Implementation

Chassis is 1.5mm steel.

Binding posts for 2:1 or 4:1 output. Heatsink is 250mm high.

 

Heatsink is mounted away from chassis with washers.

 

Transformer leads left long for testing.

Bias using ST L200 in current regulator mode (6R8 series resistor for 100 mA), a pair of Schottky diodes and third mosfet. Potentiometer across both diodes for adjusting bias.

Power mosfets mounted either side of the bias temperature tracking mosfet, L200 on right. Thermal switch on left shorts out lower diode and reduces bias level to lower quiescent power being dissipated.

A second thermal switch with a higher temperature limit could be used across both diodes to bring bias down to a minimum - which should be discernable to the ear.

Component values

Q1-2 are STW34NB20 PowerMESH MOSFETs, mounted on a heatsink of 300 x 250 mm with 80 mm fins MINIMUM. (See http://www.st.com if above link does not work.)

The bias voltage is generated from a temperature tracking supply using an additional MOSFET, schottky diode and constant current source (e.g. LM317 or L200).

Bias is set for between 500 mA to 750 mA per device - dependant on the size of the heatsink being used, the DC supply voltage, and the likely peak ambient temperature in use.

Match MOSFETS as well as possible for closest voltage drop across Drain-Source. Use a variation of the bias generator for this. See: Matching MOSFETS page for schematic and picture.

Power Supply

This is a separate unit to minimise magnetic interaction between PSU and Amplifier.

Note the relatively small reservoir capacitor. Larger or multiple capacitors may be used as per normal power amplifier practise but this is not required for the Zeus push-pull power stage circuit arrangement.

Internal view of Assembly.

Specification of unit shown:

• Transformer 250 VA, 12+12 Vac; e.g. Farnell # 9530720.
• Main Supply Reservoir Capacitor 10,000 uF (105 deg C), e.g. Farnell # 4704939.
• Secondary Supply Reservoir Capacitor 4,700 uF
• Cliff 4 pole power connector.
• Chassis is Schroff 1.5mm steel.

I have included the binding posts for development use. For normal use the power supply is via the 4 way power connector.

• Main Supply = 36 volts (off load), fused 5A slow-blow.
• Secondary Supply = 17 volts, fused 500mA semi-delay.

For 75 Watts (in 2:1 mode) into 8 ohms the power supply needs to be about 40 volts DC, which is above the direct voltage rating of the LM317 regulators and at the maximum for L200 regulators. Using a 15+15 Vac transformer with the centre tap method derives a safe half volts supply for the bias circuits.

Heatsinks

An adequate size of heatsink is very important to ensure adequate thermal stability and dissipation for any power amplifier.

Note: Please ensure that the maximum temperature that the heatsink attains during operation at is 50° Centigrade maximum. Above this temperature there is a risk of burns.

Recommended Aavid Thermalloy heatsink profile OS245, black anodized.

Use 250 mm length for Zeus 75 as illustrated in above image.

Aavid Thermalloy sales locations here.

Alternative Heatsink Profiles

Any large heatsink (assembly) should be fine as long as it is capable of 0.3°C/W or less.

Suggested minimum Heatsink Size/Type for High Bias Current Amplifier - 300 x 200 x 80 mm.

Illustrated above is a type SK158 profile heatsink from DAU Components Ltd (rendered from profile with TurboCAD V10 Pro) (This is the DAU equivalent to OS245). N.B. 200 mm length is the minimum recommended size for high bias versions of the amplifier, 250 mm (or greater) is preferred.

Thermal Pads

Good thermal contact between the semiconductor device and the heatsink is very important to ensure proper operation and longevity of the power amplifier.

My recommendation is Bergquist Sil-Pad1500ST which has excellent characteristics and has good performance with moderate contact pressure so is well suited to hand assembly.

Design by: Susan Parker, MIET.

The information contained here may be used to construct one system specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies.